BAY CITY — Was Heidi L. Lewis still a substitute teacher when she allegedly had sex with three Bay City Central High School students? The answer to that question is proving a bone of contention between attorneys in Lewis’ trial on six counts of third-degree criminal sexual conduct.

Joseph Buggs, assistant principal of Central High School, testified this morning that Lewis, 43, was a long-term substitute teacher from January 2010 through the end of the school year. Three of Lewis’ former students testified Wednesday that they had sex with her individually or two at a time during the summer between their junior and senior years, when they would have been 17.

Questioned by Bay County Assistant Prosecutor Jordan E. Case, Buggs said he considered Lewis an employee of Bay City Public Schools throughout the summer of 2010.

“To my knowledge, she was coming back in the fall,” Buggs said. Lewis taught at T.L. Handy Middle School in the fall of 2010 and eventually returned to Central High School in September, Buggs continued.

While school policy doesn’t forbid staff from socializing with students after hours, faculty members are expected to intervene if they witness students engaging in illegal activity, Buggs added.

Defense attorney Richard L. Lee Jr. argued Lewis was never employed by the school system, but by Professional Contract Management Inc., a distinct agency from which Bay City Public Schools solicits substitute teachers.

“Bay City Public Schools did not pay Heidi Lewis,” Buggs said when questioned by Lee, though the witness did say he felt she was employed by the school system.

Buggs did not answer several of Lee’s questions regarding whether or not Lewis had a personal file or the manner in which she was paid, saying such matters fall within the scope of the school system’s Human Resources division.

The age of consent for sexual intercourse in Michigan is 16, but it is increased to 18 if the participants are a student and a teacher, substitute teacher or administrator of the school attended by the student. Lee has argued that even if Lewis had sex with the teens, such conduct would not be illegal as she was not their substitute teacher when the alleged sex occurred.